1. Field of the Invention
The present invention generally relates to a means for inhibiting radio frequency (RF) emissions from a cathode ray tube (CRT) display face and, more particularly, to an RF emission inhibiting means located at the rear surface of the CRT envelope.
2. Description of the Prior Art
The large aperture usually required by a CRT display in the wall of an otherwise well-shielded enclosure presents a potential shielding deficiency. Internally generated electromagnetic energy may be transmitted through such an aperture with relatively minor attenuation. RF emissions have generally been combatted by using a transparent conducting screen to cover the face of the CRT. The conductive screen may comprise either a wire mesh or a thin homogenous layer. The screen is bonded along its entire periphery to the enclosure wall, thus completing a conducting envelope surrounding all potential radiators.
Unfortunately wire meshes and homogeneous layers have practical limitations when the restrictions on RF emission are severe. Brightness and spatial resolution penalties result with the use of a wire mesh. Moreover, annoying moire patterns are formed as a consequence of the interaction of the geometric periodicity of the mesh and the raster scan lines or dot matrix CRT phosphor geometry. A homogenous conductive film generally comprises either metallic gold or indium-tin-oxide (ITO). The homogenous conductive film must be made thicker to increase shielding effectiveness. Optical transparency of the homogenous conductive film diminishes with increased thickness and therefore a compromise on thickness is usually made which is neither satisfactory in terms of transparency nor satisfactory in terms of RF emission shielding.
The inherent problems with a transparent conducting screen for shielding RF emissions from a CRT display face can be anticipated to have an ever increasing impact on the electrical and optical performance of display systems of evolving bandwidth, optical resolution, and physical size. Transparent conducting screens have a high material cost and require a high labor cost because of the delicate bonding operations.
U.S. Pat. No. 2,217,409 to Hepp is directed to a CRT control apparatus and discloses sheathing the deflection coils with conductive material comprising wound wire. An electrostatic screen made of helically wound non-magnetic material separates the inner pair of deflection coils from the outer pair. The whole of the coil system is surrounded by a helically wound sheath of magnetic material which closes the lines of force and is coaxial with the neck of the CRT. U.S. Pat. No. 3,824,515 to Holman shows a cylindrical electric screening used in the deflection units of CRTs. The electric cage comprises two adjacent helically wound wires with one end of each wire being connected to ground and the grounded ends being located at opposite ends of the neck tube of the CRT.
U.S. Pat. No. 2,567,874 to Cage and U.S. Pat. No. 2,623,923 to Zimmerman employ Faraday cages comprising longitudinal conductors disposed in an essentially cylindrical geometry. The longitudinal conductors are shown on the inside of a CRT envelope in the Cage patent. The purpose of Cage's arrangement is to provide a better electron return path within the CRT so that higher accelerating potentials can be applied resulting in increased brightness. The arrangement also shields the electron beam from exterior electric fields. The patent to Zimmerman is directed to an electrostatically shielded magnetic well logging system. The Faraday cage permits electromagnetic coupling between a sensitive measuring system and earth formations adjacent a well bore and prevents electrostatic coupling.
U.S. Pat. No. 4,392,083 to Costello shows a radiation shield for a CRT neck which shields proximate conductors from neck emissions. a plurality of elongated, coaxially oriented, spaced, conductive elements are bonded to the inner surface and outer surface of a sleeve of insulating material. The inner conductive elements are offset from the outer conductive elements. A conductive ring is connected to each of the inner elements and outer elements at one end of the sleeve and the ring is connected to ground potential by a drain strap. The neck shield disclosed in the Costello patent renders electromagnetic radiation from the CRT neck greatly attenuated while the magnetic field generated by the yoke is allowed to pass through the shield with relatively low attenuation.
There is no teaching in the prior art of a means for shielding RF emissions from the display face of a CRT which entirely avoids the need for an optically transparent screen in front of the display face of the CRT. Moreover, there is no teaching which anticipates moving the RF radiation barrier from its conventional location in front of the CRT face to the rear surface of the CRT envelope.